Organic light emitting diode display

ABSTRACT

An organic light emitting diode display with improved aperture ratio includes: a substrate; first and second pixels disposed in a first row of the substrate and third and fourth pixels disposed in a second row adjacent to the first row and respectively disposed in the same columns as the first and second pixels; a scan line and a previous scan line applying a scan signal and a previous scan signal, respectively, to the pixel units; a data line and a driving voltage line applying a data signal and a driving voltage, respectively, to the pixel units; and a common initialization voltage line disposed between the first and second pixels and between the third and fourth pixels, commonly connected to the pixel units, and applying an initialization voltage. One common initialization contact hole connected to all pixels units and one initialization voltage line connected to the common initialization contact hole are surrounded by the pixel units.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C. §119 from an applicationearlier filed in the Korean Intellectual Property Office on the 29 ofMar. 2012 and there duly assigned Serial No. 10-2012-0032386.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an organic light emittingdiode (OLED) display. More specifically, the invention relates to anOLED display which provides an improvement in aperture ratio whileincreasing storage capacitance.

2. Description of the Related Art

An organic light emitting diode (OLED) display includes two electrodesand an organic emission layer disposed between the two electrodes, andelectrons injected from one electrode and holes injected from the otherelectrode are combined in the organic emission layer so that excitonsare formed, and light is emitted by energy generated from the excitons.

In each pixel of the OLED display, a plurality of switching thin filmtransistors are formed to compensate for non-uniformity of a thresholdvoltage of driving thin film transistors and deterioration of an organiclight emitting diode. In this case, an initialization voltage thatinitializes the driving thin film transistor is applied to the drivingthin film transistor through an initialization voltage line.

Furthermore, in order to improve visibility of the OLED display, a greenpixel having excellent visibility is further provided, and thus an OLEDdisplay of four color pixels has been applied, and the four color pixelscan be arranged according to various types such as a stripe type, achecker type, or pentile type.

In particular, in the four color pixel OLED display, initializationvoltage lines are respectively formed in two pixels in one row and intwo pixels in an adjacent row, and in this case, each pixel shouldinclude an initialization contact hole for connection of the drivingthin film transistor of each pixel with the initialization voltage line.

However, pixels of a high resolution OLED display tend to be down-sized,and accordingly a space for a storage capacitor is reduced due to aninitialization contact hole formed in each pixel, and an aperture ratiois also decreased due to the initialization contact hole formed in eachpixel.

The above information disclosed in this Background section is only forenhancement of an understanding of the background of the describedtechnology, and therefore it may contain information that does not formthe prior art that is already known in this country to a person ofordinary skill in the art.

SUMMARY OF THE INVENTION

The present invention has been developed in an effort to provide anorganic light emitting diode (OLED) display that can improve an apertureratio while increasing storage capacitance

An OLED display according to an exemplary embodiment may include: asubstrate; pixel units including first and second pixels disposed in afirst row of the substrate, and third and fourth pixels disposed in asecond row that is adjacent to the first row and respectively disposedin the same columns as the first and second pixels; a scan line and aprevious scan line respectively applying a scan signal and a previousscan signal to the pixel units; a data line and a driving voltage linecrossing the scan line and the previous scan line, and respectivelyapplying a data signal and a driving voltage to the pixel units; and acommon initialization voltage line disposed between the first and secondpixels and between the third and fourth pixels, commonly connected tothe pixel units, and applying an initialization voltage.

The OLED display may further include: a contact hole connectionelectrode formed in the same layer where the data line and drivingvoltage line are formed and surrounded by the pixel units; and aprotective layer covering the data line, the driving voltage line, andthe contact hole connection electrode. The common initialization voltageline may be formed on the protective layer, and the commoninitialization voltage line may be connected to the contact holeconnection electrode of the protective layer through a commoninitialization contact hole surrounded by the pixel units.

The OLED display may further include: a contact hole connectionsemiconductor layer formed on the substrate and overlapping the contacthole connection electrode; a gate insulating layer formed on the contacthole connection semiconductor layer; and an interlayer insulating layerformed between the previous scan line which is formed on the gateinsulating layer and the contact hole connection electrode.

The contact hole connection semiconductor layer may be connected to thecontact hole connection electrode through an auxiliary contact holeformed in the interlayer insulating layer.

The contact hole connection semiconductor layer may be connected toinitialization thin film transistors respectively formed in the first,second, third, and fourth pixels.

The scan line may include a first scan line applying a first scan signalto the first and second pixels and a second scan line applying a secondscan signal to the third and fourth pixels, and the first scan line andthe second scan line may be separated from each other with reference tothe common initialization voltage line.

The previous scan line may include a first previous scan line applying afirst previous scan signal to the first and second pixels and a secondprevious scan line applying a second previous scan signal to the thirdand fourth pixels, and the first previous scan line and the secondprevious scan line may be adjacent to each other.

The data voltage line may include a first data line applying a firstdata signal to the first and third pixels and a second data lineapplying a second data signal to the second and fourth pixels, and thefirst data line and the second data line may be symmetrical relative toeach other with reference to the common initialization contact hole.

The driving voltage line may include a first driving voltage lineapplying a first driving voltage to the first and third pixels and asecond driving voltage line applying a second driving voltage to thesecond and fourth pixels, and the first driving voltage line and thesecond driving voltage line may be symmetrical relative to each otherwith reference to the common initialization contact hole.

The first pixel and the second pixel may be symmetrical relative to thethird and fourth pixels with reference to the common initializationvoltage line.

The first and third pixels may be symmetrical relative to the second andfourth pixels with reference to the common initialization hole.

According to the present invention, one common initialization contacthole connected to all of the pixels units and one initialization voltageline connected to the common initialization contact hole are disposed ina location surrounded by the pixel units so that the initializationcontact hole may not need to be formed in each pixel, thereby improvingan aperture ratio.

In addition, the initialization contact hole may not need to be formedin each pixel, and accordingly a space for the storage capacitor can beassured.

Accordingly, the size of the pixel can be reduced, and thus the pixel isapplicable to a high resolution OLED display.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, in which likereference symbols indicate the same or similar components, wherein:

FIG. 1 is a pixel equivalent circuit diagram of an organic lightemitting diode (OLED) according to an exemplary embodiment of thepresent invention.

FIG. 2 is a layout view of a pixel unit of the OLED display according tothe exemplary embodiment of the present invention.

FIG. 3 is a cross-sectional view of the OLED display of FIG. 2, takenalong the line III-III.

FIG. 4 is an enlarged layout view of a portion of a commoninitialization contact hole of the OLED display of FIG. 2.

FIG. 5 is a cross-sectional view of FIG. 2 and FIG. 4, taken along theline V-V.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described more fully withreference to the accompanying drawings, in which exemplary embodimentsof the invention are shown. As those skilled in the art will realize,the described embodiments may be modified in various different ways, allwithout departing from the spirit or scope of the present invention.

The drawings and description are to be regarded as illustrative innature and not restrictive. Like reference numerals designate likeelements throughout the specification.

Furthermore, the size and thickness of the elements shown in thedrawings are provided for better understanding and ease of description,and the present invention is not necessarily limited thereto.

In the drawings, the thickness of layers, films, panels, regions, etc.are exaggerated for clarity. Furthermore, the thickness of some layersand regions are exaggerated in the drawings for better understanding andease of description. It will be understood that, when an element such asa layer, film, region, or substrate is referred to as being “on” anotherelement, it can be directly on the other element or intervening elementsmay also be present.

An organic light emitting diode (OLED) display according to an exemplaryembodiment of the invention will be described in further detail withreference to FIG. 1 thru FIG. 5.

FIG. 1 is an equivalent circuit diagram of one pixel of an OLED displayaccording to an exemplary embodiment of the present invention.

As shown in FIG. 1, each pixel of the OLED display according to theexemplary embodiment includes a plurality of signal lines 121, 122, 123,171, 172, and 193, a plurality of thin film transistors T1, T2, T3, T4,T5, and T6 respectively connected to the plurality of signal lines,capacitors Cst and Cb, and an organic light emitting diode OLED.

The thin film transistors include a driving thin film transistor T1, aswitching thin film transistor T2, a compensation thin film transistorT3, an initialization thin film transistor T4, a first light emissioncontrol thin film transistor T5, and a second light emission controlthin film transistor T6, and the capacitors Cst and Cb include a storagecapacitor Cst and a boosting capacitor Cb.

The signal lines include a scan line 121 transmitting a scan signal Sn,a previous scan line 122 transmitting a previous scan signal Sn−1 to theinitialization thin film transistor T4, a light emission control line123 transmitting a light emission control signal En to the first lightemission control thin film transistor T5 and the second light emissioncontrol thin film transistor T6, a data line 171 crossing the scan line121 and transmitting a data signal Dm, a driving voltage line 172transmitting a driving voltage ELVDD and formed substantially inparallel with the data line 171, and a common initialization voltageline 193 transmitting an initialization voltage Vint that initializesthe driving thin film transistor T1.

A gate electrode of the switching thin film transistor T2 is connectedto the scan line 121, a source electrode of the switching thin filmtransistor T2 is connected to the data line 171, a drain electrode ofthe switching thin film transistor T2 is electrically connected to asource electrode of the driving thin film transistor T1 and the drivingvoltage line 172. The switching thin film transistor T2 performsswitching operation according to a scan signal transmitted through thescan line 121.

The driving thin film transistor T1 receives a data signal according tothe switching operation of the switching thin film transistor T2 andsupplies a driving current to the organic light emitting diode OLED.

A gate electrode of the driving thin film transistor T1 is connected toa first end of the storage capacitor Cst, and a second end of thestorage capacitor Cst is connected to the driving voltage line 172. Inaddition, a gate electrode of the switching thin film transistor T2 isconnected to a first end of the boosting capacitor Cb, and a second endof the boosting capacitor Cb is connected to a gate electrode of thedriving thin film transistor T1.

A drain electrode of the driving thin film transistor T1 is electricallyconnected to an anode of the organic light emitting diode OLED. Inaddition, a cathode of the organic light emitting diode OLED isconnected to a common voltage ELVSS. Thus, the organic light emittingdiode OLED displays an image by emitting light with a driving currenttransmitted from the driving thin film transistor T1.

Hereinafter, an operational process of the pixel of the OLED displayaccording to the exemplary embodiment will be described in furtherdetail.

First, while the initialization thin film transistor T4 is in theturn-on state according to the previous scan signal Sn−1 transmittedthrough the previous scan line 122, the first end of the storagecapacitor Cst and the gate electrode of the driving thin film transistorT1 are supplied with the initialization voltage Vint.

Next, the switching thin film transistor T2 and the compensation thinfilm transistor T3 are turned on according to the scan signal Sntransmitted through the scan line 121. While the switching thin filmtransistor T2 and the compensation thin film transistor T3 are in theturn-on state, the data signal Dm transmitted through the data line 171is transmitted to the source electrode of the driving thin filmtransistor T1 and the driving thin film transistor T1 isdiode-connected. Then, the gate electrode and the source electrode ofthe driving thin film transistor T1 are supplied with a voltage obtainedby subtracting a threshold voltage of the driving thin film transistorT1 from the data voltage.

Next, the first light emission control thin film transistor T5 and thesecond light emission control thin film transistor T6 are turned on bythe light emission control signal En transmitted through the lightemission control line 123, and a voltage at the gate electrode of thedriving thin film transistor T1 is boosted by an increase in the scansignal Sn transmitted through the scan line 121.

While the first light emission control thin film transistor T5 and thesecond light emission control thin film transistor T6 are in the turn-onstate, the driving voltage ELVDD of the driving voltage line 172 issupplied to the source electrode of the driving thin film transistor T1and a driving current according to a voltage difference between the gateelectrode and the source electrode flows to the driving thin filmtransistor T1. The driving current is transmitted to the anode of theorganic light emitting diode OLED through the turn-on second lightemission control thin film transistor T6 so that the organic lightemitting diode OLED emits light.

A pixel unit, wherein four of the pixels of the OLED display shown inFIG. 1 are provided, will now be described in further detail withreference to FIG. 2 thru FIG. 5.

FIG. 2 is a layout view of the OLED display according to the exemplaryembodiment of the present invention, and FIG. 3 is a cross-sectionalview of the OLED display of FIG. 2, taken along the line III-III.

As shown in FIG. 2 and FIG. 3, the OLED display according to theexemplary embodiment includes a substrate 110, pixel units R, Ga, B, andGb formed of four color pixels and formed on the substrate 110, a scanline Scn and a previous scan signal Sn−1 formed along a row directionand respectively applying the scan signal Sn and the previous scansignal Sn−1 to the pixel units R, Ga, B, and Gb, a data line 171 and adriving voltage line 172 crossing the scan line 121 and the previousscan line 122 and respectively applying the data signal Dm and thedriving voltage ELVDD, and a common initialization voltage line 193disposed along a low direction between the pixel units R, Ga, B, and Gband commonly applying the initialization voltage Vint to the pixel unitsR, Ga, B, and Gb.

The pixel units R, Ga, B, and Gb include a first pixel, a second pixel,a third pixel, and a fourth pixel. The first and second pixels aredisposed in the first row and the third and fourth pixels are disposedin the second row which is adjacent to the first row. In the presentexemplary embodiment, the first, second, third, and fourth pixelsrespectively correspond to a red pixel R, a first green pixel Ga, a bluepixel B, and a second green pixel Gb. Thus, hereinafter, it will bedescribed that the pixel units R, Ga, B, and Gb include a red pixel R, afirst green pixel Ga, a blue pixel B, and a second green pixel Gb.

The blue pixel B is disposed in the same column as the red pixel R andthe second green pixel Gb is disposed in the same column as the firstgreen pixel Ga. Therefore, the red pixel R, the first green pixel Ga,the blue pixel B, and the second green pixel Gb form a checker-typepixel units R, Ga, B, and Gb.

The scan line 121 includes a first scan line 121 a applying a first scansignal to the red pixel R and the first green pixel Ga, and a secondscan line 121 b applying a second scan signal to the first scan line 121a and to the blue pixel B and the second green pixel Gb. In addition,the previous scan line 122 includes a first previous scan line 122 aapplying a first previous scan signal to the red pixel R and the firstgreen pixel Ga, and a second previous scan line 122 b applying a secondprevious scan signal to the blue pixel B and the second green pixel Gb.

The first scan line 121 a and the second scan line 121 b are separatedfrom each other by a constant gap with reference to the commoninitialization voltage line 193, and the first previous scan line 122 aand the second previous scan line 122 b are adjacent to each other.

The data line 171 includes a first data line 171 a applying a first datasignal to the red pixel R and the blue pixel B, and a second data line171 b applying a second data signal to the first green pixel Ga and thesecond green pixel Gb. In addition, the driving voltage line 172includes a first driving voltage line 172 a applying a first drivingvoltage to the red pixel R and the blue pixel B, and a second drivingvoltage line 172 b applying a second driving voltage to the first greenpixel Ga and the second green pixel Gb.

The first data line 171 a and the second data line 171 b are disposedsymmetrically relative to each other with reference to a commoninitialization contact hole 185, and the first driving voltage line 172a and the second driving voltage line 172 b are symmetrical relative toeach other with reference to the common initialization contact hole 185.

The common initialization voltage line 193 is disposed between the redpixel R and the first green pixel Ga and between the blue pixel B andthe second green pixel Gb, and is commonly connected to theinitialization thin film transistors T4 of each of the pixel units R,Ga, B, and Gb.

Each of the red pixel R, the first green pixel Ga, blue pixel B, and thesecond green pixel Gb includes the driving thin film transistor T1, theswitching thin film transistor T2, the compensation thin film transistorT3, the initialization thin film transistor T4, the first light emissioncontrol thin film transistor T5, the second light emission control thinfilm transistor T6, the storage capacitor Cst, the boosting capacitorCb, and the organic light emitting diode 70.

The structures of the thin film transistors formed in the red pixel Rand the blue pixel B will be described in detail, and the structures ofthe thin film transistors formed in the first green pixel Ga and thesecond green pixel Gb are substantially equivalent to the structures ofthe thin film transistors formed in the red pixel R and the blue pixelB.

The driving thin film transistor T1 includes a driving semiconductorlayer 131 a, a driving gate electrode 125 a, a driving source electrode176 a, and a driving drain electrode 177 a. The driving source electrode176 a corresponds to a driving source area of the driving semiconductorlayer 131 a, and the driving drain electrode 177 a corresponds to adriving drain area of the driving semiconductor layer 131 a.

The switching thin film transistor T2 includes a switching semiconductorlayer 131 b, a switching gate electrode 125 b, a switching sourceelectrode 176 b, and a switching drain electrode 177 b.

The compensation thin film transistor T3 includes a compensationsemiconductor layer 131 c, a compensation gate electrode 125 c, acompensation source electrode 176 c, and a compensation drain electrode177 c, and the initialization thin film transistor T4 includes aninitialization semiconductor layer 131 d, an initialization gateelectrode 125 d, an initialization source electrode 176 d, and aninitialization drain electrode 177 e.

The first light emission control thin film transistor T5 includes afirst light emission control semiconductor layer 131 e, a first lightemission control gate electrode 125 e, a first light emission controlsource electrode 176 e, and a first light emission control drainelectrode 177 e, and the second light emission control thin filmtransistor T6 includes a second light emission control semiconductorlayer 131 f, a second light emission control gate electrode 125 f, asecond light emission control source electrode 176 f, and a second lightemission control drain electrode 177 f.

The storage capacitor Cst includes a first capacitor plate 132 and asecond capacitor plate 127 with a gate insulating layer 140 interposedtherebetween. Here, the interlayer insulating layer 140 becomes adielectric material, and capacity is determined by the amount of chargecharged in the storage capacitor Cst and a voltage between the twocapacitor plates 132 and 127.

The driving semiconductor layer 131 of the driving thin film transistorT1 connects the switching semiconductor layer 131 b and the compensationsemiconductor layer 131 c to each other, and connects the first lightemission control semiconductor layer 131 e and the second light emissioncontrol semiconductor layer 131 f to each other.

Thus, the driving source electrode 176 a is connected to the switchingdrain electrode 177 b and the first light emission control drainelectrode 177 e, and the driving drain electrode 177 a is connected tothe compensation drain electrode 177 c and the second light emissioncontrol source electrode 176 f.

The first capacitor plate 132 of the storage capacitor Cst is connectedto the compensation source electrode 176 c and the initialization drainelectrode 177 d, and the second capacitor plate 128 is connected to thecommon voltage line 193 and is formed substantially in parallel with thescan line 121.

The first capacitor plate 133 of the boosting capacitor Cb is connectedto the driving gate electrode 125 a through a connection member 174, andthe second capacitor plate 129 is a portion protruding from the scanline 121.

The switching thin film transistor T2 is used as a switching element forselecting a pixel for light emission. The switching gate electrode 125 bis connected to the scan line 121. The switching source electrode 176 bis connected to the data line 171. The switching drain electrode 177 bis connected to the driving thin film transistor T1 and the first lightemission control thin film transistor T5.

The second light emission control drain electrode 177 f of the secondlight emission control thin film transistor T6 is directly connected toa pixel electrode 191 of the organic light emitting diode 70 through acontact hole 181 of a protective layer 180.

The red pixel R and the first green pixel Ga including the thin filmtransistors T1, T2, T3, T4, T5, and T6 are symmetrical relative to theblue pixel B and the second green pixel Gb with reference to the commoninitialization voltage line 193.

In addition, the red pixel R and the blue pixel B are symmetricalrelative to the first green pixel Ga and the second green pixel Gb withreference to the common initialization contact hole 185.

Hereinafter, the structure of the OLED display according to theexemplary embodiment will be described according to a layering sequencewith reference to FIG. 3.

In particular, the structure of a thin film transistor will be describedbased on the second light emission control thin film transistor T6. Inaddition, the other thin film transistors T1, T2, T3, T4, and T5 arealmost the same in layering structure as the second light emissioncontrol thin film transistor T6, and therefore no further description ofthe thin film transistors T1, T2, T3, T4, and T5 will be provided.

A buffer layer 111 is formed on the substrate 110, and the second lightemission control semiconductor layer 131 f is formed on the buffer layer111. The substrate 110 is formed of an insulating substrate made ofglass, quartz, ceramic, plastic, or the like. The second light emissioncontrol semiconductor layer 131 f is formed of a polysilicon layer. Inaddition, the second light emission control semiconductor layer 131 fincludes a channel area that is not doped with an impurity, and a sourcearea and a drain area that are p+ doped at both sides of the channelarea. Here, the impurity may be changed according to the type of thinfilm transistor.

A gate insulation layer 140 formed of silicon nitride (SiNx) or siliconoxide (SiO₂) is formed on the second light emission controlsemiconductor layer 131 f.

The scan line 121 including the switching gate electrode 125 b and thecompensation gate electrode 125 c, the previous scan line 122 includingthe initialization gate electrode 125 d, the driving gate electrode 125a, and the light emission control line 123 including the second lightemission control gate electrode 125 f are formed on the gate insulatinglayer 140. The second light emission control gate electrode 125 f isformed so as to overlap with at least a part, in particular, the channelarea, of the second light emission control semiconductor layer 131 f.The gate wire further includes a second capacitor plate 127 forming astorage capacitor. The second capacitor plate 127 is connected to thedriving voltage line 172 through the contact hole 168.

An interlayer insulating layer 126 which covers the second lightemission control gate electrode 125 e is formed on the gate insulatinglayer 140. The gate insulating layer 140 and the interlayer insulatinglayer 160 include a contact hole 163 exposing a drain area of the secondlight emission control semiconductor layer 131 f. Like the gateinsulating layer 140, the interlayer insulating layer 160 is formedusing a ceramic-based material such as silicon nitride (SiNx) or siliconoxide (SiO2).

A data wire, including the data line 171 that includes the switchingsource electrode 176 b, the connection member 174, the second lightemission control drain electrode 177 f, and the driving voltage line172, is formed on the interlayer insulating layer 160.

In addition, the switching source electrode 176 b and the second lightemission control drain electrode 177 f are respectively connected to thesource area of the switching semiconductor layer 131 b and the drainarea of the second light emission control semiconductor layer 131 fthrough the contact holes 162 and 163, respectively, formed in theinterlayer insulating layer 160 and the gate insulating layer 140.

The protective layer 180, which covers the data wires 171, 174, 177 f,and 172, is formed on interlayer insulating layer 160, and the pixelelectrode 191 is formed on the protective layer 180. The pixel electrode191 is connected to the second light emission control drain electrode177 f through the contact hole 181 formed in the protective layer 180.

A barrier rib 350 is formed on an edge of the pixel electrode 191 andthe protective layer 180, and the barrier rib 350 includes a barrier ribopening 351 that exposes the pixel electrode 191. The barrier rib 350may be formed of a resin such as a polyacrylates resin and polyimides ora silica-based inorganic material.

An organic emission layer 370 is formed on the pixel electrode 191exposed through the barrier rib opening 351, and a common electrode 270is formed on the organic emission layer 370. Accordingly, the organiclight emitting diode 70, including the pixel electrode 191, the organicemission layer 370, and the common electrode 270, is formed.

Here, the pixel electrode 191 is an anode which is a hole injectionelectrode, and the common electrode 270 is a cathode which is anelectron injection electrode. However, the exemplary embodiment of theinvention is not limited thereto, and the pixel electrode 191 may be acathode and the common electrode 270 may be an anode according to thedriving method of the OLED display. Holes and electrons are injectedfrom the pixel electrode 191 and the common electrode 270 into theorganic light emitting layer 370, and when exitons, formed as the holesand electrons injected into the organic light emitting layer 370, arecombined and change from a base state to an excited state, the organiclight emitting layer 370 emits light.

The organic emission layer 370 is formed of a low molecular organicmaterial or a high molecular organic material such as Poly3,4-ethylenedioxythiophene (PEDOT). In addition, the organic emissionlayer 370 may be formed of a multilayer including at least one of anemission layer, a hole injection layer (HIL), a hole transport layer(HTL), an electron transport layer (ETL), and an electron injectionlayer (EIL). When the organic emission layer 370 includes all theabove-stated layers, the hole injection layer (HIL) is disposed on thefirst electrode 71 which is a hole injection electrode, and the holetransport layer (HTL), the emission layer, the electron transport layer(ETL), and the electron injection layer (EIL) are sequentially layeredthereon. Since the common electrode 270 is formed of a reflectiveconductive material, the OLED display is formed as a bottom emissiontype OLED display. The reflective material may include lithium (Li),calcium (Ca), fluoride lithium/calcium (LiF/Ca), fluoridelithium/aluminum (LiF/Al), aluminum (Al), silver (Ag), magnesium (Mg),and gold (Au).

Hereinafter, the common initialization contact hole formed in a locationsurrounded by the pixel units will now be described in further detail.

FIG. 4 is an enlarged layout view of a portion of the commoninitialization contact hole of FIG. 2, and FIG. 5 is a cross-sectionalview of FIG. 2 and FIG. 4, taken along the line V-V.

As shown in FIG. 4 and FIG. 5, the buffer layer 111 is formed on thesubstrate 110, and a contact hole connection semiconductor layer 135 isformed on the buffer layer 111. The contact hole connectionsemiconductor layer 135 includes four external connection branches 135R,135Ga, 135B, and 135Gb and two internal connection branches 1351 and1352. The four external connection branches 135R, 135Ga, 135B, and 135Gbare respectively connected with the initialization semiconductor layer131 of each of the pixel units R, Ga, B, and Gb.

The gate insulating layer 140 is formed on the contact hole connectionsemiconductor layer 135, the previous scan line 122 b is formed on thegate insulating layer 140, and interlayer insulating layer 160 is formedon previous scan line 122 b. A contact hole connection electrode 175 isformed on the interlayer insulating layer 160. The contact holeconnection electrode 175 is surrounded by the pixel units R, Ga, B, andGb, and is formed in the same layer where the data line 171 and thedriving voltage line 172 are formed. The two internal connectionbranches of the contact hole connection semiconductor layer 135 areconnected to the contact hole connection electrode 175 through anauxiliary contact hole 169 formed in the interlayer insulating layer160.

The protective layer 180 is formed on the contact hole connectionelectrode 175 and the interlayer insulating layer 160, and the commoninitialization voltage line 193 is formed on the protective layer 180.One common initialization contact hole 185 which exposes the contacthole connection electrode 175 is formed in the protective layer 180. Thecommon initialization contact hole 185 is formed in a locationsurrounded by the pixel units R, Ga, B, and Gb, and the commoninitialization voltage line 193 is connected to the contact holeconnection electrode 175 through the common initialization contact hole185.

Thus, when the previous scan signal is transmitted through the previousscan line 122, the initialization thin film transistor T4 of each of thepixel units R, Ga, B, and Gb connected to the previous scan line 122 isturned on. In this case, an initialization voltage transmitted throughthe common initialization voltage line 193 is transmitted to the contacthole connection electrode 175 through the common initialization contacthole 185, and subsequently, the initialization voltage is transmitted tothe contact hole connection semiconductor layer 135 through theauxiliary contact hole 169. Thus, the initialization voltage is suppliedto the gate electrode of the driving thin film transistor T1 of each ofthe pixel units through the four external connection branches 135R,135Ga, 135B, and 135Gb so as to initialize the driving thin filmtransistor T1.

As described, one common initialization contact hole 185 connected toall of the pixel units R, Ga, B, and Gb and one common initializationvoltage line 193 connected to the common initialization contact hole 185are formed in a location surrounded by the pixel units R, Ga, B, and Gbso that an initialization contact hole may not need to be formed in eachpixel, thereby improving the aperture ratio.

In addition, the initialization contact hole may not need to be formedin each pixel, and accordingly a space for the storage capacitor can beassured.

Accordingly, the size of the pixel can be reduced, and thus the pixel isapplicable to a high resolution OLED display.

While this disclosure has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. An organic light emitting diode (OLED) displaydevice, comprising: a substrate; pixel units including first and secondpixels disposed in a first row of the substrate, and third and fourthpixels disposed in a second row adjacent to the first row, said firstand third pixels are located in a first column and said second andfourth pixels are located in a second column with said first columnbeing adjacent to said second column, said first row is a separate anddifferent row from said second row and said first column is a separateand different column from said second column; a scan line and a previousscan line for applying a scan signal and a previous scan signal,respectively, to the pixel units; a data line and a driving voltage linecrossing the scan line and the previous scan line for applying a datasignal and a driving voltage, respectively, to the pixel units; a commoninitialization voltage line disposed between the first and second row,said common initialization voltage line disposed between the first andthird pixels and between the second and fourth pixels, and said commoninitialization voltage line commonly connected to the pixel units, forapplying an initialization voltage; and a contact hole connectionelectrode formed in a same layer as the data line and driving voltageline are formed, and said contact hole electrode is electricallyconnected to the pixel units and surrounded by the pixel units; whereinthe common initialization voltage line is connected to the contact holeconnection electrode through a common initialization contact holesurrounded by the pixel units, and wherein said common initializationvoltage line applies the initialization voltage to each of the first,second, third and fourth pixels via the contact hole and the contacthole connection electrode.
 2. The OLED display device of claim 1,further comprising: a protective layer covering the data line, thedriving voltage line, and the contact hole connection electrode; andwherein the common initialization voltage line is formed on theprotective layer.
 3. The OLED display device of claim 2, furthercomprising: a contact hole connection semiconductor layer formed on thesubstrate and overlapping the contact hole connection electrode; a gateinsulating layer formed on the contact hole connection semiconductorlayer and having a previous scan line formed thereon; and an interlayerinsulating layer formed between the previous scan line that is formed onthe gate insulating layer and the contact hole connection electrode. 4.The OLED display device of claim 3, wherein the contact hole connectionsemiconductor layer is connected to the contact hole connectionelectrode through an auxiliary contact hole formed in the interlayerinsulating layer.
 5. The OLED display device of claim 3, wherein thecontact hole connection semiconductor layer is connected toinitialization thin film transistors respectively formed in the first,second, third, and fourth pixels.
 6. The OLED display device of claim 5,wherein the first pixel and the second pixel are symmetrical relative tothe third and fourth pixels with reference to the common initializationvoltage line.
 7. The OLED display device of claim 6, wherein the firstand third pixels are symmetrical relative to the second and fourthpixels with reference to the common initialization hole.
 8. The OLEDdisplay device of claim 3, wherein the first pixel and the second pixelare symmetrical relative to the third and fourth pixels with referenceto the common initialization voltage line.
 9. The OLED display device ofclaim 8, wherein the first and third pixels are symmetrical relative tothe second and fourth pixels with reference to the common initializationhole.
 10. The OLED display device of claim 9, wherein the contact holeconnection semiconductor layer is connected to the contact holeconnection electrode through an auxiliary contact hole formed in theinterlayer insulating layer.
 11. The OLED display device of claim 10,wherein the contact hole connection semiconductor layer is connected toinitialization thin film transistors respectively formed in the first,second, third, and fourth pixels.
 12. The OLED display device of claim2, wherein the first pixel and the second pixel are symmetrical relativeto the third and fourth pixels with reference to the commoninitialization voltage line.
 13. The OLED display device of claim 12,wherein the first and third pixels are symmetrical relative to thesecond and fourth pixels with reference to the common initializationhole.
 14. The OLED display device of claim 2, wherein the scan linecomprises a first scan line applying a first scan signal to the firstand second pixels and a second scan line applying a second scan signalto the third and fourth pixels, and wherein the first scan line and thesecond scan line are separated from each other with respect to thecommon initialization voltage line.
 15. The OLED display device of claim14, wherein the previous scan line comprises a first previous scan lineapplying a first previous scan signal to the first and second pixels anda second previous scan line applying a second scan signal to the thirdand fourth pixels, and wherein the first previous scan line and thesecond previous scan line are adjacent to each other.
 16. The OLEDdisplay device of claim 15, wherein the data voltage line comprises afirst data line applying a first data signal to the first and thirdpixels and a second data line applying a second data signal to thesecond and fourth pixels, and wherein the first data line and the seconddata line are symmetrical relative to each other with reference to thecommon initialization contact hole.
 17. The OLED display device of claim16, wherein the driving voltage line comprises a first driving voltageline applying a first driving voltage to the first and third pixels anda second driving voltage line applying a second driving voltage to thesecond and fourth pixels, and wherein the first driving voltage line andthe second driving voltage line are symmetrical relative to each otherwith reference to the common initialization contact hole.